The invention described herein relates generally to apparatus and method involving magnetocumulative generators, i.e., to apparatus and method in which work is done on magnetic fields by the compression of the magnetic flux thereof by moving conductors.
At the outset, some fundamental principles of the physics underlying magnetocumulative generators will be given. A magnetic field may be completely characterized by a vector function B which is variously called the magnetic induction, the magnetic flux density, the magnetic field strength, or in recent years simply the magnetic field. Because of the non-existence of magnetic charge, the divergence of B is everywhere zero. The most familiar unit of B is the gauss. The intensity of the earth's magnetic field, near the surface of the earth, is of the order of one-half gauss. Magnetic fields of up to about 80,000 gauss can be supplied by commercially available superconducting magnets. However, magnetic fields greater than about 100,000 gauss are difficult to achieve. Magnetic field B should not be confused with the vector function H, the unit of which is the oersted. The vector function H is equal to B minus 4.pi.M, where M is the magnetic moment density. H does not necessarily have zero divergence. Any magnetic field possesses magnetic flux. The quantity of magnetic flux passing through or linking a closed curve is given by the surface integral of B over any surface which has the closed curve for its boundary. Magnetic flux is thus measured in units of B times area, such as gauss cm.sup.2. Because the divergence of B equals zero, as much flux enters any volume as leaves it. It is frequently helpful to conceptualize tubes of flux. A flux tube is bounded by a surface which is everywhere tangent to B. Therefore no flux passes through the surface of a flux tube. Consequently a flux tube can be thought of as containing a definite amount of flux. Both the magnetic energy density and the magnetic hydrostatic pressure at points within a magnetic field are proportional to B.sup.2.
Known magnetocumulative generators consist of a housing defining an internal housing chamber which may be partially open. The surface of the chamber is electrically conducting. The chamber is filled with magnetic flux so that the flux is surrounded by electrical conductor on all sides except those perpendicular to the direction of the magnetic field. The magnetic flux is usually created by means external to the housing chamber which employ a high voltage and current electrical source, such as a capacitor bank. The flux is often introduced into the housing chamber via a slot in the conducting surface of the chamber. When the quantity of magnetic flux, or the magnitude of the magnetic field, reaches a prescribed value the slot is closed and external forces are applied to the housing which cause a collapse of the conducting surface of the housing chamber. This collapse compresses the magnetic flux so that the value of the magnetic field becomes greatly increased. In most applications the enhanced magnetic field is utilized within the housing chamber itself. As a specific example of a known magnetocumulative generator, a perspective cutaway view of a typical early model discussed by A.D. Sakharov, Usp. Fiz. Nauk 88, 725 (1966), is shown in FIG. 1. It is of a type which has been in use at least since 1952, and consists of hollow metallic cylinder 10 within which a longitudinal magnetic field is established by means of the discharge of capacitor bank 12 through solenoid winding 14 which surrounds the cylinder. A narrow slot 16 cut into the cylinder insures rapid penetration of the field. At about the time when the current in the solenoid winding is at a maximum, a converging cylindrical shock wave is produced in the metallic cylinder by means of the detonation of explosive charge 18. The explosive charge is set off either by an electric multiple-point initiation system or by detonation lenses. The contraction process rapidly closes narrow slot 16. To first approximation ohmic losses in the cylinder are so small that the cylinder behaves as an ideal conductor so that the magnetic flux enclosed within the contracting volume does not change appreciably. In even the first experiments fields of one million gauss were obtained by means of magnetocumulative generators of this type. However these very high fields were, and to date have been, achieved only over very small spatial regions. In prior art magnetocumulative generators of this type, in the region of flux compression, if no flux is lost into the enclosing conductor, then to first order, the magnetic field B and the total magnetic energy of the magnetic field E both increase in proportion to the compression ratio. This is the ratio, in the region of compression, of the initial volume filled by the magnetic field V.sub.o to the final volume filled by the magnetic field V. The average magnetic pressure P increases in proportion to the compression ratio squared. These relationships may be expressed:
B=B.sub.o (V.sub.o /V), PA1 P=P.sub.o (V.sub.o /V).sup.2, PA1 E=E.sub.o (V.sub.o /V),
where the subscript zero denotes initial value. Alternatively, a magnetocumulative generator may be regarded as an electrical circuit whose inductance varies under the influence of external forces. Neglecting electrical resistance and other causes of flux loss, the magnetic flux .phi. is a constant equal to the product of the prevailing values of the inductance L and the electrical current I producing the field. Again allowing the subscript zero to denote initial value, this may be expressed. EQU .phi.=I.sub.o L.sub.o =IL.
The energy in the circuit is given by EQU E=1/2LI.sup.2 =E.sub.o (L.sub.o /L).
The magnetocumulative generator shown in FIG. 1 increases the value of a magnetic field by compressing its associated magnetic flux tube. A perspective view of part of a typical magnetic flux tube 20 such as could be produced by the solenoid winding 14 of FIG. 1 is shown in FIG. 2. Magnetic field lines 22 which lie in the surface of the flux tube are shown. As schematically shown in FIG. 3, known magnetocumulative generators such as that shown in FIG. 1 function by rapidly compressing a flux tube 25 so that the energy density and the pressure of the magnetic field are greatly increased in the region of constriction. Although it would be ideal if the flux tube could be contained within a perfect conductor, in actuality a large amount of magnetic flux is lost during the operation of presently existing magnetocumulative generators. In other words, when a hypothetical perfect conductor compresses a flux tube electrical currents are induced in the surface of the perfect conductor which maintain the total flux within the flux tube at a constant value. But, since conductors are not perfect, some work expended in compressing a flux tube quickly degenerates into heat in the conductor. This causes some induced currents to die away thereby allowing some flux to escape through the conductor surface. This effect is known as resistive diffusion. Because of resistive diffusion and material strength limitations under conditions of high magnetic pressure, it is characteristic of the performance of presently existing magnetocumulative generators that, while they can easily achieve an increase in magnetic energy by the factor ten, they can achieve an increase in magnetic energy by the factor one-hundred only with great difficulty. This fundamental limitation in the amount of magnetic energy which magnetocumulative generators are currently able to produce has seriously limited their efficacy as tools of research. Although magnetocumulative generators are presently being studied at laboratories in the United States and several foreign countries, most of this work is directed toward the achievement of very high magnetic fields in relatively small spatial regions. If magnetocumulative generators were capable of producing appreciably greater quantities of magnetic energy over larger volumes their importance to presently existing fields of research would be increased, and their use could be extended to many applications which are impossible at the present time. One such use would be to supply very high power to cyclic inductive elementary particle accelerators.